A humanized antibody can be prepared by replacing the regions in a mouse antibody that are not critical for antigen specificity with a human counterpart. The resulting recombinant antibodies thus have residual murine sequences which, when administered to a human patient, often elicit immunological responses in the patient (human anti-murine response). Therefore, it is desirable to prepare fully human antibodies that are void of non-human sequences. Fully human antibodies have been reportedly obtained, such as by constructing and screening a human antibody library using the phage display technology, by grafting lymphocytes from immunized human donors into severe combined immunodeficient (SCID) mice, or by engineering transgenic mice harboring human immunoglobulin genes. Fully humanized human antibodies against pathogens can also be obtained by isolation of umbilical cord blood containing a natural polyreactive IgM repertoire through extensive screening (see, for example, U.S. Pat. No. 6,391,635). These methods, however, either produce antibodies with low affinities or depend on human donors with a desired immune response.
Respiratory syncytial virus (RSV, also known as syncytial virus, also belonging to Paramyxoviridae) is the most common cause of viral pneumonia in children, which may cause interstitial pneumonia and bronchiolitis. 48% of viral pneumonia and 58% of bronchiolitis in Beijing (1980-1984), 31.4% of pneumonia and bronchiolitis in children in Guangzhou (1973-1986) and 20% to 25% of infant pneumonia and 50% to 75% of bronchiolitis in the United States are caused by the syncytial virus.
The RSV infection has an incubation period of 2 to 8 days (mostly 4 to 6 days). Typical pathological changes of syncytial virus pneumonia are interstitial infiltrates of monocytes, which are mainly characterized by widened alveolar septa and interstitial effusion, mainly monocytes including lymphocytes, plasma cells, and macrophages. In addition, the alveolar space is filled with edema fluid and the formation of a transparent membrane in the lung can be observed. In some cases, lymphocytic infiltration of the bronchiole wall can be further observed. The presence of edema in the lung parenchyma with necrotic areas leads to alveolar packing, consolidation, and collapse. In a few cases, multinucleated fusion cells can be observed in the alveolar space, whose morphology is similar to that of measles giant cells, but no nuclear inclusion bodies could be found.
RSV has two major surface glycoproteins, F and G. The two glycoproteins (90 KDa and 68 KDa) respectively are presented on the surface of the virus particles. The highly glycosylated G protein, 90 KDa, is responsible for the binding of viral particles to host cells. The glycoprotein F, 68 KDa, mediates viral cell membrane fusion and syncytia formation. The surface glycoproteins F and G are the major protective antigens, while nucleoprotein N and envelope protein M2 have less protective antigenicity. Compared with monoclonal antibodies against glycoprotein F, monoclonal antibodies against glycoprotein G are less likely to neutralize viruses and have no fusion inhibiting activity. The amino acid sequence of the glycoprotein F is approximately 90% conserved among the RSV strains associated with human infection.
The sole anti-RSV monoclonal antibody currently on the market is only approved for the prevention of RSV infection in preterm infants, which is a humanized monoclonal antibody against the glycoprotein F with name of Palivizumab Synagis (Made by MedImmune). The drug prevents the virus from spreading to the lower respiratory tract through respiratory syncytial virus fusion protein (glycoprotein F).